1. Field of the Invention
The invention relates in general to solid freeform fabrication and, in particular, to a layer normalizing device for use in producing parts by selective deposition modeling techniques. The layer normalizing device utilizes capillary action to wick excess build material away from the part.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies can generally be described as solid freeform fabrication, herein referred to as xe2x80x9cSFFxe2x80x9d. In SFF, complex parts are produced from a modeling material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. Generally, SFF technologies, such as stereolithography and the like, utilize a computer graphic representation of a part and a supply of a building material to fabricate the part in successive layers.
One category of SFF that has recently emerged is selective deposition modeling, herein referred to as xe2x80x9cSDMxe2x80x9d. In SDM, a solid modeling material is physically deposited in successive fashion to form an object. In one type of SDM technology the solid modeling material is extruded as a continuous filament through a resistively heated nozzle. In yet another type of SDM technology the solid modeling material is jetted or dropped in discrete droplets in order to build up a part. Often, a thermoplastic material having a low-melting point is used as the solid modeling material, which is delivered through a jetting system such as those used in ink jet printers. One type of SDM process utilizing ink jet print heads is described, for example, in U.S. Pat. No. 5,555,176 to Menhennett, et al. Another type of SDM process which extrudes a bead of material to build a part is described, for example, in U.S. Pat. No. 5,303,141 to Batchelder et al.
Although SFF methods have many advantages compared to conventional fabrication methods, they also have inherent problems routed in the layer by layer building process. One common problem in the layer by layer building process results from the dimensional variability inherent in the building of each layer. These dimensional inaccuracies occur from the accumulated effects of drop volume variation, thermal distortion, and the like. If unchecked, these tolerances can accumulate throughout the part as it is built up in height layer by layer. As the thickness of layers are reduced to achieve greater surface resolution, the accumulated buildup of these undesirable tolerances can substantially distort the resultant part. Thus, most SDM processes require some method to dimensionally normalize the part while it is being built. Generally, all dimensional normalization methods involve physically adjusting the vertical height of the part by smoothing or leveling the build material deposited in the layers. One common approach is to dispense each layer of build material at a greater thickness than desired so that the normalizing device can then remove the excess build material to achieve the desired thickness and thereby eliminate undesirable distortions between the layers. Alternatively, some methods do not dimensionally normalize each layer but normalize only after a certain number of layers have been deposited. Still other methods selectively normalize a layer after receiving instructions from active sensor controls monitoring the build process.
One approach to providing a system to dimensionally normalize a part while being built by an SDM apparatus is found in U.S. Pat. No. 5,943,235 to Earl et al., wherein a pre-heated rotating planarizer is provided to normalize each layer. Under this approach, after a layer of build material has been deposited by the SDM apparatus in excess of the necessary amount to achieve a desired thickness, the pre-heated cylindrical roller (planarizer) is precisely passed over the deposited material. The rolling planarizer locally melts some of the build material that adheres to its surface as it rolls to thereby dimensionally normalize the deposited layer to conform to the desired thickness of the layer. A wiping or scraping device such as a blade is needed to peel or skive off the excess build material from the planarizer. Undesirably, the rolling planarizer must be manufactured to precise tolerances in order to achieve the desired accuracy. It is also difficult to precisely regulate and maintain the temperature of the surface of the planarizer. This is due in part because the planarizer is subject to non-uniform convection heat transfer as it rotates, and the heater element only provides a uniform delivery of heat through the rotating axis of the planarizer. The planarizer is also thermally inefficient as it consumes a significant amount of energy that is undesirably dissipated into the environment. The planarizer also occupies a significant amount of space within the SDM apparatus and thereby limits the over-travel distance of the dispensing carriage. The planarizer also has moving parts that are subject to wear and degradation. Airborne contaminants are also prone to accumulate on the planarizer. In short, the heated rotating planarizer adds significant cost and complexity to an SDM apparatus, occupies precious space, adds inertia, is subject to wear, and requires maintenance and adjustment.
Other approaches to providing a system to dimensionally normalize a part while being built by an SDM apparatus is found in U.S. Pat. No. 5,859,775 to Barlage, III et al. and U.S. Pat. No. 5,572,431 to Brown et al. Under these approaches, a heated body is selectively driven across the dispensed build material in response to a sensed deviation in order to melt and displace the build material. These approaches also suggest providing a vacuum source connected to the heated body to actively suck off excess build material through the heated body. Thus, dimensional normalization is discretely accomplished in response to a sensed deviation monitored by the system. Such active monitoring/normalizing methods undesirably add additional complexity and cost to the SDM apparatus. In addition, without a wiping system the heated body is prone to undesirably accumulate build material on its normalizing surface. Such accumulation can adversely impact dimensional normalization operations.
Thus, there is a need to provide an SDM process with an improved layer normalizing device capable of dimensionally normalizing parts as they are built layer by layer. There is also a need to provide a layer normalizing device that is inexpensive and requires a minimal amount of maintenance and repair. There is also a need to provide a layer normalizing device which withdraws excess build material from the surface of the object being built without allowing the build material to accumulate on the surface of the device. These and other difficulties of the prior art have been overcome according to the present invention.
The present invention provides its benefits across a broad spectrum of SFF processes. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As will be understood, the basic apparatus and methods taught herein can be readily adapted to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
It is one aspect of the present invention to provide a new layer normalizing device for use in an SDM apparatus capable of being maintained at a more uniform temperature.
It is another aspect of the present invention to provide a new layer normalizing device for use in an SDM apparatus that removes excess build material from the surface of the object being built, without the application of mechanical forces.
It is yet another aspect of the present invention to provide a new layer normalizing device that removes excess build material from the surface of the object being built while preventing the build material from adhering to the normalizing surface of the device.
It is a feature of the present invention to utilize capillary action to wick excess build material from the surface of the object instead of relying on mechanical forces such as wiping and scraping.
It is another feature of the present invention to dimensionally normalize an object being built with a heated porous material having interstices that provide the geometric configuration necessary to initiate the capillary action to wick the excess build material from the surface of the object.
It is still another feature of the present invention to provide a material refreshing means to expel build material from the interstices of the normalizing device in order to prevent the interstices from reaching a point of saturation where the capillary action would terminate.
It is an advantage of the present invention to dimensionally normalize each layer of an object built by an SDM apparatus with a simple layer normalizing device that has no moving parts and can be uniformly heated and maintained within a desired temperature range.
It is another advantage of the present invention that material can be removed from the surface of an object being built by an SDM apparatus without the need for mechanical operations such as skiving, peeling, swiping, scraping, or the like.
It is yet another advantage of the present invention to produce a layer normalizing device that has no rotating parts, that is inexpensive to produce, and that occupies a minimum amount of space within an SDM apparatus.
These and other aspects, features, and advantages are achieved/attained in the apparatus of the present invention that employs a wicking member having a normalizing surface and a plurality of interstices for dimensionally normalizing the object while the object is being built layer by layer. The wicking member is sufficiently heated to cause the build material that it contacts to change to a flowable state, and due to capillary action of the interstices of the wicking member, the excess build material from the layer of the object being normalized is drawn into the wicking member. A refreshing means is provided to remove the excess build material from the interstices of the wicking member in order to allow the wicking member to draw additional build material by capillary action into the interstices.